This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The Holland group from the University of Rochester's Department of Chemistry utilized the X-Band EPR instrumentation at ACERT to study iron compounds with hydride and sulfide donors. These studies were part of an on-going investigation of the spectroscopy of coordination compounds with the same coordination features as the iron-molybdenum cofactor of nitrogenase. These are intended to facilitate the use of spectroscopy to understand the enzyme. The continuous-flow, liquid helium cryostat available at ACERT is essential for these experiments because the EPR spectra of cyts c are typically not observed above 30 K due to a relatively short spin-lattice relaxation time.